Some wireless telecommunication systems include a Multiple Access Control (MAC) protocol controlling how a plurality of users share a common communication medium, e.g. a set of radio channels such as in FDMA, OFDMA, TDMA, and CDMA. Desired characteristics of such a MAC protocol include
(i) low delays for transmitted data,
(ii) a high aggregate throughput or capacity, and
(iii) service differentiation, so that e.g. high priority services can be guaranteed also for very high traffic loads.
MAC protocols can be divided into
conflict-free, or “scheduled”, protocols, and
contention-based, or “direct access” protocols.
Conflict-free protocols ensure that transmissions from other users within a certain set such as a geographic area, e.g. in a cell of a cellular network, do not interfere with transmissions from a considered user. When contention-based protocols are used collisions between transmissions can occur, and principles for resolving such conflicts must be defined. Contention-based protocols allow direct transmission attempts, without previous signaling to ensure that the shared medium is free. This can result in very low delays for transmissions from the users. For high loads however, the risk of collisions between transmission attempts increases, and hence the delay times increase. The collisions also result in that the shared medium is frequently occupied by non-successful transmission attempts, this in turn resulting in a poor aggregate throughput. Service differentiation can be achieved by allowing transmissions of different priorities from the users to access or try to access the shared medium with different probabilities.
Conflict-free protocols involve some signaling before data is transmitted to ensure that the transmission will not be in conflict with other transmissions. Although the duration of this signaling phase may be short, the resulting delay may represent a large fraction of the total transmission time for transmissions from the users, in particular for transmitting small amounts of data. For large amounts of data the duration of the signaling phase is less important. One benefit of conflict-free protocols is that, as opposed to contention-based protocols, a full use of the shared medium can be achieved. This results in a high capacity potential. In the case of a central control node being in charge of the allocation of transmission opportunities, service differentiation can simply be achieved by the central node granting such opportunities based on service type or priority.
A qualitative comparison of the delay versus served traffic characteristics of the two alternatives is illustrated in the graph of FIG. 1. For low traffic loads the contention-based protocols give lower delays than the conflict-free protocols, whereas for high traffic loads, the conflict-free protocols give lower delays than the contention-based protocols.
In existing wireless network standards, conflict-free MAC protocols with service differentiation are used e.g. for GSM/GPRS and WCDMA, see 3GPP, GSM 04.60, “General Packet Radio Service (GPRS); Mobile Station (MS)—Base Station System (BSS) interface; Radio Link Control/Medium Access Control (RLC/MAC) protocol”. A user having data to transmit sends a channel request message to the network, including a priority class, and the base station responds with a resource assignment, pointing to a “reserved” or dedicated resource, including a frequency and time slot or code. This resource will not be used by any other transmission in the cell.
Contention-based protocols are used e.g. by WLANs (Wireless Local Area Networks) built according to the standard IEEE802.11. More specifically, a Carrier Sense Multiple Access (CSMA) protocol is used. Before transmitting, a user first listens to the medium for a certain time to determine that it is not busy. If the medium is busy, or if the transmission from the user fails, the user waits a random backoff time before repeating the process. In the standard IEEE 802.11e, the medium sensing and backoff times are service-class dependent. Contention-based protocols are also used in many cellular systems, e.g., WCDMA, for initiating a call and requesting resources using a random access procedure.
WLANs built according to the standard IEEE 802.11 also have a conflict-free mode. In this mode a central “point coordinator” can take control of the medium and grant transmission opportunities to users in a conflict-free way. The conflict-free and contention-based modes may co-exist, sharing the transmission medium in the time domain. Then, the point coordinator controls the fractions of the medium used by each mode, as well as which users are allowed to transmit in the conflict-free mode. There is no way to control which users that are allowed to use the contention-based mode.
As seen in FIG. 1, the minimum delays are achieved with contention-based MAC protocols in low load traffic situations.